This invention relates to an electronic monitoring and redundant control circuit for a load carrying power switch which may be a semiconductor switch, for example, a semiconductor switch in a solid state relay for switching AC or DC load current. The load carrying power switch may also be an electromechanical relay or contractor or a controlled mechanical switch.
Generally, solid state switching devices for switching AC load current use, for example, a triac or a pair of thyristors in inverse parallel connection, as a load carrying switch. This switch is controlled, e.g., by an input current signal applied through an optocoupler. For turning off the load carrying switch, the control signal is removed, and the power switch turns off near the next zero crossing of the AC current. However, problems may occur, for example, with switching off inductive loads, which shift the phase between voltage and current by up to ninety degrees.
In these cases, the semiconductor power switch, in particular a triac, may fail to turn off reliably. In any case, independent of the kind of load, a semiconductor switch can be defective. Due to a failure of the semiconductor switch, AC or DC load current can continue to flow even in the absence of an input control signal.
Similar problems can also arise with electromechanical switching devices, since mechanical contacts sometimes can weld closed.